1. Field of the Invention
The present invention relates generally to servo circuits and more particularly to a servo circuit suitable for use in controlling a drum motor of, for example, a video tape recorder.
2. Description of the Prior Art
In a video tape recorder (VTR) in which a rotary head is mounted on a rotary head drum, a servo circuit is generally used to stabilize the running speed of a tape, i.e., a recording medium or to stabilize the revolution rate of a rotary head (see Japanese Laid-Open Patent Gazette No. 61-39877 and so on).
FIG. 1 illustrates an example of such a conventional servo circuit that controls a rotary head drum.
Referring to FIG. 1, there is shown a rotary head drum 1, and this rotary head drum 1 is rotated by a motor 2. A frequency generator 3 and a pulse generator 4 are mounted on the peripheral surface of the rotary head drum 1. The frequency generator 3 is adapted to generate a frequency signal FG of, for example, 720 Hz or to generate a signal each time the rotary head drum 1 is rotated by a predetermined angle (for example, 15.degree.). The pulse generator 4 is adapted to generate a pulse signal of, for example, 30 Hz or to generate a pulse signal at every revolution of the rotary head drum 1.
The signal FG from the frequency generator 3 and the signal PG from the pulse generator 4 are both supplied to a servo signal processor section 5 that is represented by a phantom line in FIG. 1. This servo signal processor section 5 might be formed of an integrated circuit (IC). The signal FG supplied to the servo signal processor section 5 is supplied to a cycle detector circuit 6 therein. The cycle detector circuit 6 detects the revolution speed of the rotary head drum 1 on the basis of the signal FG and produces a voltage signal corresponding to the detected signal indicative of the revolution speed of the rotary head drum 1. The signal PG supplied to the servo signal processor section 5 is supplied to a first comparing signal input terminal of a phase difference detector circuit 7 therein. A reference signal generator 8 in the servo signal processor circuit 5 is adapted to generate a reference signal V.sub.D. This reference signal V.sub.D is supplied to a second comparing signal input terminal of the phase difference detector circuit 7. When the video tape recorder is operated in the recording mode, the reference signal generator 8 receives through its terminal 9 the vertical synchronizing signal of a video signal to be recorded and generates the reference signal V.sub.D synchronized with the vertical synchronizing signal. When the video tape recorder is operated in the playback mode, a mode changing signal is supplied through a terminal 8a to the reference signal generator circuit 8 to disconnect the terminal 9 therefrom and the reference signal generator 8 itself generates the reference signal V.sub.D. Then, the phase difference detector circuit 7 detects a phase difference between the pulse signal PG and the reference signal V.sub.D and produces a voltage signal corresponding to a detected phase difference therebetween.
The voltage signal from the cycle detector circuit 6 is supplied to a plus (+) input terminal of a subtracter circuit 10. Also, a voltage signal corresponding to reference speed data obtained at a terminal 11 is supplied to a minus (-) input terminal of the subtracter circuit 10. Thus, the subtracter circuit 10 subtracts the voltage signals through the terminal 11 from the voltage signal of the detector circuit 6 to produce a subtracted voltage signal that indicates a difference between the reference speed and a real speed. The subtracted signal is supplied to a first adding signal input terminal of an adder circuit 12 and the voltage signal from the phase difference detector circuit 7 is supplied to a second adding signal input terminal of the adder circuit 12. Thus, the adder circuit 12 adds the subtracted signal and the voltage signal to each other to produce an added signal of the speed difference and the phase difference. Then, the added signal from the adder circuit 12 is supplied to a phase compensator circuit 13 and is thereby phase-compensated. The output signal from the phase compensator circuit 13 is supplied to a driver circuit 14 and this driver circuit 14 drives the drum motor 2 to rotate the rotary head drum 1.
When the drum motor 2 is servo-controlled in this way, in the recording mode of the video tape recorder, the rotary head drum 1 is controlled to rotate in synchronism with the video signal to be recorded, while upon reproduction the rotary head drum 1 is controlled to rotate in synchronism with the reference signal produced within the video tape recorder.
However, if the pulse signal PG of about 30 Hz from the pulse generator 4 is used to detect the phase difference, there is then a disadvantage that the rotary head drum 1 cannot be rotated stably without a relatively lot of time.
Further, when in the playback mode the video tape recorder performs the variable tape speed reproduction such as a high-speed reproduction and so on, the revolution speed of the rotary head drum 1 has to be varied. In this case, the frequency of the reference signal V.sub.D from the reference signal generator circuit 8 has to be varied, making the reference signal generator circuit 8 complicated in arrangement.